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This paper investigates in an innovative way the climatological vertical stratification of relative humidity (RH), ozone (O-3) and carbon monoxide (CO) mixing ratios within the planetary boundary layer (PBL) and at the interface with the free troposphere (FT). The climatology includes all vertical profiles available at northern midlatitudes over the period 1994-2016 in both the IAGOS (In-service Aircraft for a Global Observing System) and WOUDC (World Ozone and Ultraviolet Radiation Data Centre) databases, which represents more than 90 000 vertical profiles. For all individual profiles, apart from the specific case of surface-based temperature inversions (SBIs), the PBL height is estimated following the elevated temperature inversion (EI) method. Several features of both SBIs and EIs are analysed, including their diurnal and seasonal variations. Based on these PBL height estimates (denoted h), the novel approach introduced in this paper consists of building a so-called PBL-referenced vertical distribution of O-3, CO and RH by averaging all individual profiles beforehand expressed as a function of z/h rather than z (with z the altitude). Using this vertical coordinate system allows us to highlight the features existing at the PBL-FT interface that would have been smoothed otherwise.

Results demonstrate that the frequently assumed wellmixed PBL remains an exception for both chemical species. Within the PBL, CO profiles are characterized by a mean vertical stratification (here defined as the standard deviation of the CO profile between the surface and the PBL top, normalized by the mean) of 11 %, with moderate seasonal and diurnal variations. A higher vertical stratification is observed for O-3 mixing ratios (18 %), with stronger seasonal and diurnal variability (from similar to 10% in spring-summer midday-afternoon to similar to 25% in winter-fall night). This vertical stratification is distributed heterogeneously in the PBL with stronger vertical gradients observed at both the surface (due to dry deposition and titration by NO for O-3 and due to surface emissions for CO) and the PBL-FT interface. These gradients vary with the season from the lowest values in summer to the highest ones in winter. In contrast to CO, the O-3 vertical stratification was found to vary with the surface potential temperature following an interesting bell shape with the weakest stratification for both the lowest (typically negative) and highest temperatures, which could be due to much lower O-3 dry deposition in the presence of snow.

Therefore, results demonstrate that EIs act as a geophysical interface separating air masses of distinct chemical composition and/or chemical regime. This is further supported by the analysis of the correlation of O-3 and CO mixing ratios between the different altitude levels in the PBL and FT (the so-called vertical autocorrelation). Results indeed high-light lower correlations apart from the PBL-FT interface and higher correlations within each of the two atmospheric compartments (PBL and FT).

The mean climatological O-3 and CO PBL-referenced profiles analysed in this study are freely available on the IAGOS portal for all seasons and times of day (https://doi.org/10.25326/4).